I think you will need several steps: surpress NULL in dimensions and expression, enabling dimension-scroll like suggested above, use a horizontal legend: Horizontal Legend in chart and maybe some more.

Within the tab dimension you could for each dimension set different options like "Hide Null", "Show All" and within the tab presentation is a global option to hide NULL or missing values in expressions.

I'm not sure if this is relevant for the space below and above the bars but it should be worth an attempt. You could also export the chart to excel to see which content will be displayed for the space.